This invention relates generally to a ballast for ignition of a high intensity discharge lamp, and more particularly to a scheme for controlling the voltage and/or current applied to the lamp during the initial, non steady state operating modes.
In starting a high intensity discharge (HID) lamp, the lamp experiences three phases. These phases include breakdown, glow discharge, and thermionic arc. Breakdown requires a high voltage to be applied to the electrode. Following breakdown, the voltage must be high enough to sustain a glow discharge and heat the electrode to thermionic emission. Once thermionic emission commences, current must be maintained, in the run-up phase, until the electrode reaches its steady-state temperature. After achieving the arc state, the lamp can be operated with a lower level of current in the steady state operating mode.
The overall life and efficiency of a lamp are affected by this starting sequence as are the values and tolerances of the components required to effect this starting sequence. For ignition, in the pre-breakdown period, the lamp electrode must be brought to a high voltage for a specified duration. Conventional lamps are characterized by a minimum voltage level and time duration in achieving breakdown. Typical minimums range from about 2 to 3 KV for voltage and about 10-100 ms for time duration.
The high voltage requirements for breakdown can be achieved through pulse resonant circuits. The frequency at which the circuit achieves resonance and the resultant resonant voltage varies from circuit to circuit due to variation in component tolerances. Such variation results in the pulse resonant circuit being designed to withstand nominal pulse voltages of about 4 to 5 KV, that is, in the circuit being designed to withstand voltages which are well beyond the 2 to 3 KV range required to start the lamp. An undesirable increase in cost for the pulse resonant circuit can result,
Upon achieving breakdown, the lamp enters the non-thermionic glow state. In this phase, the voltage must be sufficiently high to maintain the flow of electrons. Electrons are produced by positive ion bombardment of the cathode which produces secondary electron emission. When the kinetic energy of the positive ions, determined by the cathode fall, is high enough, sputtering of the electrode occurs. Sputtering of the electrode produces volatile species of, for example, tungsten which condense on and blacken the inner surface of the lamp. As the interior of the lamp blackens, transmission of light through the envelope decreases reducing the visible light level. The pieces of tungsten which are deposited on the wall absorb radiation thereby heating the lamp wall above its optimum temperature. A reduction in lamp life can result.
A proper balance must be maintained between minimizing the glow state duration and electric field magnitude to maximize both lamp efficacy and lamp life. This balance is difficult to achieve since a decrease in the amount of energy supplied to the electrode will prolong the glow state duration while an increase in the amount of energy supplied to the electrode will shorten lamp life through an increase in sputtering.
As compared to the non-thermionic glow phase, during the thermionic arc phase, the lamp voltage is reduced and lamp current is increased. During the thermionic arc phase, residual sputtering can still occur. After applying a sufficiently high current to heat the electrode during the thermionic arc phase, the current is reduced and thereafter the lamp is operated under steady state conditions.
Accordingly, it is desirable to provide an improved HID ballast in which the variation in component tolerances can be decreased. The ballast should also provide a proper balance between minimizing the glow state duration and electric field magnitude to maximize both lamp efficacy and lamp life.